Image display employing filter coated phosphor particles

ABSTRACT

An image display comprising a viewing screen including a layer of phosphor particles emissive of light of a particular visual color and color filter particles transmissive of light of that color .Iadd.partially .Iaddend.covering .[.between 20 and 80 percent of.]. the surfaces of the phosphor particles.

BACKGROUND AND SUMMARY OF THE INVENTION

In U.S. Pat. No. 3,308,326 to S. H. Kaplan, there is disclosed a colortelevision picture tube including a viewing screen comprised of threeinterlaced patterns of target elements. One pattern is red emitting, onepattern is green emitting, and one pattern is blue emitting. In order toimprove the image contrast of the screen for viewing in relativelybright ambients, that patent suggests placing a red-transmitting colorfilter in front of the red-emitting elements, either as a separate layerin front of the red-emitting elements, or as a thin overcoating or layersurrounding each red-emitting phosphor particle. It has been found thatpractical filter layers which surround the phosphor particles absorb toomuch of the light emitted from the phosphor particles, with the resultthat the displayed image is not as bright as it could be.

The novel image display, which may be a color television picture tube,comprises a luminescent viewing screen including a layer of phosphorparticles and color filter particles adhered to the surfaces of thephosphor particles and .Iadd.partially .Iaddend.covering .[.between 20and 80 percent of.]. the surfaces of those phosphor particles. Thephosphor particles are emissive of light in a particular portion of thevisible spectrum, and the color filter particles are transmissive oflight in those portions of the spectrum and absorptive of light in otherportions of the visible spectrum. By only partially covering thephosphor particles with filter particles, the transmission, absorption,and reflection of light from different portions of the visible spectrummay be tailored to optimize the brightness and contrast of the displayedimage in relatively bright ambients.

The invention may be applied to monocolor and multicolor displays. Inmulticolor displays, the filter particles may be applied to the phosphorparticles in one or more of the constituent color elements of thedisplay. In one form of the invention, the layer offilter-particle-coated phosphor particles may be backed up with a layerof phosphor particles having no filter particle coating thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially broken away longitudinal view of a novelcathode-ray tube of the invention.

FIG. 2 is an enlarged fragment of the viewing screen of the tube shownin FIG. 1.

FIG. 3 is an idealized sectional view of a red-emitting phosphorparticle employed in the viewing screen of the tube in FIG. 1.

FIG. 4 is an idealized sectional view of a blue-emitting phosphorparticle employed in the viewing screen of the tube of FIG. 1.

FIG. 5 is an enlarged fragment of a viewing screen which may besubstituted in the tube shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The cathode-ray tube illustrated in FIG. 1 is an aperture mask typekinescope. The tube includes an evacuated envelope designated generallyby the numeral 21, which includes a neck 23 integral with a funnel 25,and a faceplate or panel 27 joined to the funnel 25 by a seal 29,preferably of devitrified glass. There is a luminescent layer 31comprised of phosphor material on the interior surface of the faceplate27. The construction of the luminescent layer 31 is considered in moredetail below in connection with FIGS. 2, 3 and 4. There is alight-reflecting metal coating 33, as of aluminum, on the luminescentlayer 31. The luminescent layer 31, when suitably scanned by threeelectron beams from a gun in a mount assembly 35 located in the neck 23,is capable of producing a luminescent image in color, which may beviewed through the faceplate 27. The luminescent layer 31, thelight-reflecting metal coating 33, and any associated structureconstitute the viewing screen of the tube.

There is an electrically-conducting internal coating 37 on a portion ofthe interior surface of the funnel 25 between the mount assembly 35 andthe seal 29. Three metal fingers 39 space the mount assembly 35 from theneck wall and connect the forward portion of the mount assembly 35 withthe internal coating 37. Closely spaced from the metal coating 33 towardthe mount assembly 35 is a metal mask 41 having a multiplicity ofapertures therein. The mask 41 is welded to a metal frame 43 which issupported by springs 47, which are attached to the frame 43, on studs 45sealed in the wall of the panel 27. Except for the details of theluminescent layer 31, the tube is conventional in construction andoperation, so that a detailed description thereof is not necessary atthis time.

FIG. 2 shows that the viewing screen of the tube of FIG. 1 includes theluminescent layer 31 comprised of separate red-emitting target elementsR1, separate green-emitting target elements G1 and separateblue-emitting target elements B1, on the inner surface of the panel 27and overlaid by the reflecting layer 33. The target elements aregenerally circular in shape and are arranged on the inner surface of thepanel 27 in a hexagonal array of three interlaced patterns, one of eachemission color. Each aperture in the mask 41 is associated with threetarget elements, called a trio, one of each emission color. The viewingscreen may be similar to the screen described in U.S. Pat. No. 3,423,621to M. R. Royce, except for the presence of filter particles on thered-emitting and blue-emitting phosphor particles.

The red-emitting target elements R1 consist principally of red-emittingphosphor particles 51 of europium-activated yttrium oxysulfide partiallycoated with red-transmitting filter particles 53, as shown in FIG. 3.The phosphor particles 51 .[.are about 8 to 12 microns average sizeand.]. have adhered to their surfaces red-transmitting filter particles53 of cadmium sulfoselenide which are about 0.1 to 0.5 microns averagesize. .[.On the average, about 40 percent of the.]. .Iadd.The.Iaddend.surfaces of the phosphor particles 51 are .Iadd.only partially.Iaddend.covered with filter particles 53.

The blue-emitting target elements B1 consist principally ofblue-emitting phosphor particles 55 of silver-activated zinc sulfidecoated with blue-transmitting filter particles 57, as shown in FIG. 4.The phosphor particles 55 .[.are about 7 to 10 microns average sizeand.]. have adhered to their surfaces blue-transmitting filter particles57 of a cobalt aluminate which are about 0.1 to 0.5 micron average size..[.On the average, about 50 percent of the.]. .Iadd.The.Iaddend.surfaces of the phosphor particles 55 are .Iadd.only partially.Iaddend.covered with filter particles 57.

The green-emitting target elements G1 consist principally ofgreen-emitting phosphor particles of copper-activated zinc-cadmiumsulfide. In this embodiment, the green-emitting phosphor particles arenot coated with filter particles.

The red-emitting target elements R1 are loosely packed layers about 1 to4 particles thick. When excited, the particles emit red light andpossibly other spectral components. The filter particles 53 absorb someof these other spectral components and also some of the red light. Thefilter particles 53 also absorb spectral components of light fromambient sources. The elements R1 exhibit a light output of about 98percent and a reflectivity of about 56 percent compared with elementswith similar visual color and no light filtering. Less than full surfacecoverage of the phosphor particles 51.[., in the range of 20 percent to80 percent of the phosphor surface,.]. permits the optimum amount ofambient light to be absorbed consistent with the transmission of anoptimum brightness of emitted red light of greater purity. Examples ofother red-emitting phosphors that may be used are manganese-activatedzinc orthophosphate, silver-activated zinc-cadmium sulfide,europium-activated yttrium vanadate, and europium-activated yttriumoxide. Examples of other red-transmitting filter materials that may beused are ruby, red-tinted silicate glasses, and red ceramic pigments.

The blue-emitting target elements B1 are loosely packed layers about 1to 4 particles thick. When excited, the phosphor particles 55 emit bluelight and possibly other spectral components. The filter particles 57absorb some of the light of these other spectral components and some ofthe blue light. The filter particles 57 also absorb spectral componentsof light from ambient sources. The elements B1 exhibit a light output ofabout 94 percent and a reflectivity of about 61 percent compared withelements with similar visual color and no light filtering. Less thanfull surface coverage.[., in the range of about 20 percent to 80percent.]. of the phosphor .[.surface,.]. .Iadd.particles.Iaddend.permits the optimum amount of ambient light to be absorbedconsistent with the transmission of an optimum brightness of emittedblue light of greater purity. Examples of other blue-emitting phosphorsthat may be used are titanium-activated calcium-magnesium silicate andterbium-activated yttrium oxysulfide. Examples of otherblue-transmitting filter materials are ultramarine, blue-tinted silicateglasses and blue ceramic pigments.

Examples of other green-emitting phosphors that may be used arewillemite, manganese-activated zinc aluminate, and silver-activatedzinc-cadmium sulfide. Where it is desired to apply a green-transmittingfilter material, one may use chromium oxide Cr₂ O₃ mixtures of yellowcadmium sulfoselenide and blue-green cobalt aluminates, green-tintedsilicate glasses and green ceramic pigments.

GENERAL CONSIDERATIONS

The image display includes a viewing screen which may becathodoluminescent, photoluminescent, electroluminescent or other formof excitation. Furthermore, the display may be a single color or amulticolor display. The elements of the display may be of any shape andsize. Where the display is multicolor, one or more of the color elementpatterns may include the color filter particles.

FIG. 2 illustrates a phosphor layer 31 in which the red-emitting andblue-emitting elements R1 and B1 are principally filter-particle-coatedphosphor particles. FIG. 5 illustrates still another phosphor layer 31awhich is comprised of a layer of filter-particle-coated phosphorparticles R2 and B2 facing the viewing surface and a layer of phosphorparticles free of filter particles R3 and B3 on the other side, for thered-emitting and blue-emitting elements respectively. The green-emittingelements G2 do not include filter particles. The double-layer elementsshown in FIG. 5 may be produced by two successive applications ofphosphor particles. An advantage of the double layers R2-R3 and B2-B3 isthat filter particles are absent from the portion of the color elementsadjacent the reflecting metal layer 33a, where filter particles are morelikely to reduce luminescent brightness and less likely to absorb lightfrom ambient sources.

If desired, the green-emitting elements G1 of FIG. 2 may consistprincipally of green-emitting phosphor particles partially coated withgreen-transmitting filter particles. Also, if desired, thegreen-emitting elements may comprise a double layer (similar to theblue-emitting elements shown in FIG. 5), one layer principally offilter-particle-coated phosphor particles facing the viewer and theother layer of uncoated phosphor particles.

In deciding whether to and how much to coat the phosphor particles, ithas to be that the percent decrease in reflectivity of the finalelements must be at least twice as large as the percent loss in lightoutput from the element for the same visual color. In the case of thegreen-emitting elements (such as shown in FIGS. 2 and 5), thisspecification is difficult but not impossible to comply with for tworeasons. First, the human eye sensitivity peaks in the green, andtherefore losses in brightness are more noticeable than reductions inreflectivity. Second, at the present stage of development,green-transmitting filter materials generally are not too efficient.

The filter material has a characteristic filtering power. The greaterthe filtering power, the lower the coverage required on the phosphorparticles. Tinted glasses are generally of lower filtering power thaninorganic compounds and therefore require a greater coverage of thephosphor particles. In practical systems, the light output should be atleast 90 percent of the unfiltered light of the same viaual color. Then,the reflectivity should be as low as possible consistent with this lightoutput.

The filter-particle-coated phosphor particles may be prepared by anyconvenient process. The process disclosed in U.S. Pat. No. 3,275,466 toR. D. Kell has been found to be a practical process for adheringcontrolled amounts of filter particles to the surfaces of phosphorparticles. That process includes several steps. The phosphor particles,which are about 5 to 20 microns in average size, are coated with anadsorptive film as by immersing the phosphor particles in a solution ofgelatin and then washing away any excess gelatin with deionized water.Then, the phosphor particles are agitated in a suspension of filterparticles, which are about 0.1 to 0.5 micron average size, in deionizedwater that is free of adsorptive material. The filter particles cling tothe phosphor particles, giving a partial coverage of the surface. Thefilter particle coated phosphor particles are then washed in deionizedwater. If the particles are not sufficiently covered, the foregoingsteps may be repeated one or more times as desired until the requiredcoverage by filter particles is built up. Then, thefilter-particle-coated phosphor particles are dried and are ready fordeposit as a phosphor layer. The filter-particle-coated phosphors can bestored as a suspension or as a dry powder.

In order to adjust the percent surface coverage of the phosphorparticles, one or more of the following expedients may be used. Toincrease the coverage, the particles may be coated two or more times. Toreduce the coverage, the concentration of filter particles in thesuspension thereof may be reduced to produce the desired coverage. Also,after coating and before drying, sonic agitation reduces the coverage bya small amount. Also, adjusting the pH of the filter-particle suspensionbetween 4.0 and 9.0 can adjust the coverage. The higher the pH, thelower the attractive strength of the gelatin-coated phosphor particles.

EXAMPLE 1

A suitable filter-coated red-emitting phosphor material may be preparedby the following procedure. The red-transmitting filter material used isa cadmium sulfoselenide marketed by Harshaw Chemical Co., Cleveland,Ohio, as No. 1550. About 225 grams of the filter material are suspendedin 800 milliliters of a 6 weight percent aqueous polyvinyl pyrrolidonesolution and milled for ten days in a polyurethane-lined mill withone-half-inch ZrO₂ radius end cylinders as the milling media. Theresultant filter-particle suspension is diluted with 9 parts by weightof distilled water. To prepare the filter-particle-coated phosphormaterial, about 225 grams of europium-activated yttrium oxysulfidephosphor are used. The phosphor powder is mixed with about 500 ml of an0.1 percent gelatin solution. The phosphor powder is allowed to settle,or is centrifuged down, through the gelatin solution, and thesupernatent liquid decanted. The settled phosphor is washed three timeswith about 500 ml distilled water and decanted. The washed phosphor ismixed with 112.5 ml of diluted filter particle suspension and agitatedfor about 10 minutes. The phosphor particles are permitted to settleuntil the supernatent liquid is clear. The settled phosphor appears tohave a uniform light red body color. The supernatent liquid is decanted,and the settled phosphor is washed twice with distilled water. At thispoint, .[.about 40 percent of.]. the surface of the phosphor particlesis .Iadd.only partially .Iaddend.covered with filter particles. Thephosphor particles are dried and may be handled as any other powderedphosphor material for producing a luminescent screen.

EXAMPLE 2

A suitable filter-coated blue-emitting phosphor material may be preparedby the following procedure. The blue-transmitting filter material usedis a cobalt aluminate marketed by Harshaw Chemical Company as No. 7546.About 225 grams of the filter material are suspended in 800 ml of a 6weight percent aqueous solution of polyvinyl pyrrolidone and milled asdescribed above for the red-emitting material, and milled for four daysin a polyurethane-lined mill. The resultant filter particle suspensionis diluted with nine parts of distilled water. To prepare the filterparticle coated phosphor material, about 225 grams of a silver-activatedzinc-sulfide phosphor are used. The phosphor powder is mixed with about500 ml of an 0.1 weight percent gelatin solution and then allowed tosettle, and the supernatent liquid decanted. The settled phosphor iswashed three times with about 500 ml distilled water and decanted. Thenabout 295 ml of the diluted suspension is added and agitated for about10 minutes. The phosphor particles are permitted to settle until thesupernatent liquid is clear. The settled phosphor particles appear tohave a uniform light blue body color. The supernatant liquid is decantedand the phosphor washed twice with distilled water. At this point,.[.about 50 percent of.]. the surface of the phosphor particles is.Iadd.only partially .Iaddend.covered with filter particles. The coatedphosphor particles may be stored as an aqueous suspension, or may bedried, and the dried phosphor may be handled as other phosphor powdersfor preparing luminescent screens.

EXAMPLE 3

A suitable filter-coated green-emitting phosphor material may beprepared by following the procedure of Example 1 except that thephosphor is a green-emitting copper-activated zinc-cadmium sulfide andthe filter material is green-transmitting chromium oxide Cr₂ O₃.

I claim: .[.
 1. An image display comprising a luminescent viewing screenand means for selectively exciting areas of said screen to luminescence,said screen comprising a layer of phosphor particles emissive of lightin a particular portion of the visible spectrum, said phosphor particleshaving adhered to their surfaces color filter particles which aretransmissive of light in said portion of the visible spectrum, saidcolor filter particles covering between 20 and 80 percent of thesurfaces of said phosphor particles..]. .[.2. The display means definedin claim 1 wherein said viewing screen comprises also a second layer ofphosphor particles on said layer of filter-particle-coated phosphorparticles on the side thereof opposite the viewing side of said screen,said second layer being free of said color filter particles..]. .[.3.The image display defined in claim 1 consisting essentially of acathode-ray tube comprising a luminescent viewing screen and means forselectively exciting areas of said screen to luminescence with anelectron beam..]. .[.4. A multicolor display comprising a luminescentviewing screen including at least two interlaced patterns of targetelements, each pattern of which emits light of a particular color in thevisible spectrum upon excitation, and means for selectively excitingeach of said elements to emission, each of the elements of one of saidpatterns being comprised of a layer of phosphor particles which, uponexcitation, emit light of a particular color in the visible spectrum,and color filter particles adhered to the surfaces of said phosphorparticles, said color filter particles being transmissive of light ofsaid particular color and being absorptive of light of other colors, andcovering between 20 percent and 80 percent of the surfaces of saidphosphor particles..]. .[.5. The multicolor display defined in claim 4wherein each of said elements of said one pattern is comprised of alayer of red-emitting phosphor particles and red-transmitting colorfilter particles adhered to the surfaces of said red-emitting phosphorparticles..]. .[.6. The multicolor display defined in claim 4 whereinsaid phosphor particles are of europium-activated yttrium oxysulfide andsaid color filter particles are of cadmium sulfo-selenide..]. .[.7. Themulticolor display defined in claim 4 wherein each of said elements ofsaid one pattern is comprised of a layer of blue-emitting phosphorparticles and blue-transmitting color filter particles adhered to thesurfaces of said blue-emitting phosphor particles..]. .[.8. Themulticolor display defined in claim 7 wherein said phosphor particlesare of silver-activated zinc sulfide and said color filter particlescobalt aluminate..]. .[.9. The multicolor display defined in claim 4wherein said luminescent viewing screen comprises three interlacedpatterns of target elements, one pattern of which is red emitting, onepattern of which is green emitting and one pattern of which is blueemitting..]. .[.10. The multicolor display defined in claim 9 whereineach of the elements of said red-emitting pattern is comprised of alayer of red-emitting phosphor particles and red-transmitting colorfilter particles adhered to the surfaces of said red-emitting phosphorparticles, and each of the elements of said blue-emitting pattern iscomprised of a layer of blue-emitting phosphor particles andblue-transmitting color filter particles adhered to the surfaces of saidblue-emitting phosphor particles..]. .Iadd.11. An image displaycomprising a luminescent viewing screen and means for selectivelyexciting areas of said screen to luminescence, said screen comprising alayer of phosphor particles emissive of light in a particular portion ofthe visible spectrum, said phosphor particles having adhered to theirsurfaces color filter particles which are transmissive of light in saidportion of the visible spectrum, said color filter particles partiallycovering the surfaces of said phosphor particles, said phosphorparticles and said filter particles being present in such proportionsthat the percent decrease in reflectivity from said layer is at leasttwice as large as the percent loss in luminescent light output from saidlayer for the same visual color as compared with a similar layer that isfree from filter particles. .Iaddend. .Iadd.12. An image displaycomprising a luminescent viewing screen and means for selectivelyexciting areas of said screen to luminescence, said screen comprising alayer of phosphor particles emissive of light in a particular portion ofthe visible spectrum, said phosphor particles having adhered to theirsurfaces color filter particles which are transmissive of light in saidportion of the visible spectrum, said color filter particles partiallycovering the surfaces of said phosphor particles, said phosphorparticles and said filter particles being present in such proportionsthat the percent decrease in reflectivity from said layer is at leasttwice as large as the percent loss in luminescent light output from saidlayer for the same visual color as compared with a similar layer that isfree from filter particles, and wherein the luminescent light outputfrom said layer of the same visual color is at least 90 percent of thelight outpt of a similar layer that is free from filter particles..Iaddend.